1. Field of the Invention
The present invention relates to a solid state television camera which uses a semiconductor element, for example, a charge coupled device as an image sensor, and especially to a solid state television camera which has means to greatly reduce noise caused by defects in the semiconductor crystal of the image sensor.
2. Description of the Prior Art
Solid state television cameras have been proposed in the prior art which use charge coupled devices (hereinafter referred to as CCDs) as solid state image sensors.
The CCD is constructed by forming a plurality of electrodes separated by a predetermined pitch on an insulating layer formed over all of one surface of a semiconductor substrate. The optical image of an object projected on the image sensor from the side of the electrodes or from the other side of the substrate. The optical image generates charge carriers under the electrodes in the substrate corresponding to the image of the object. The charge carriers under the electrodes are transferred by clock pulses applied to the electrodes, and read out successively from the CCD.
Various CCD imagers are known, so the detailed explanation of the CCD imagers will not be given in this specification.
Since in the CCDs, it is rather difficult to form the crystal of the semiconductor uniform over a predetermined area, crystal defects often are produced locally, and electric charges are apt to be produced at such defects by thermal causes, which cause an abnormally large dark current. As a result, when the camera using the above CCD picks up an object whose image is projected on the CCD, noise appears in an output picked up signal, at portions where the dark current is abnormally large. Then as shown in FIG. 1, the noise N is mixed in the picked up output signal SA and appears in the reproduced picture which is undesirable.
In the prior art, in order to remove such noise, it is proposed to control a sampling hold circuit to which the picked up output signal of CCD is supplied, by the output of a comparator which compares a level of the output signal of CCD with a predetermined level, for example, a level higher than a white level Ew as shown in FIG. 1. In this case, if the level of the output signal of CCD is higher than a predetermined level, the signal of the comparator holds the sampling hold circuit, thus, any abnormally high level output signal of CCD is not read out.
One improvement of the solid state television camera with noise eliminating function explained above is shown in copending United States patent application, Ser. No. 790,090, filed on Apr. 22, 1977, and assigned to the same assignee as the present application.
The level of the noise N which is caused by the defect depends greatly on the condition of the defect of semiconductor crystal. Some of them exceed the white level as described above, and some have a middle level between the white level and zero level, namely a grey level, where a normal output signal exists also. Thus, in the above prior art television camera, which compares the level of the output signal of the CCD with the predetermined level, noise exceeding the white level can be removed from the output signal of the CCD, but the noise of the grey level can't be removed.